I have sold tons of clothing (womens, maternity & kids) to thredUp and have been very happy with the results. It's all stuff I would've been getting rid of anyway, so to get a few bucks for it is great. I wish the turnaround time for processing my clothing was faster though.
As for purchasing, thredUp is AWESOME for kids clothing. For the same price you'd pay at Target, you can get high quality, name-brand items that look brand new. I've also been pretty happy with the items I've purchased for myself (mostly shirts), but it's hit or miss with the cut and sizing. But with free returns (if you choose to receive a store credit), it's ok to risk it and order the item even if you're not sure it will work.
Overall, I recommend thredUp and have been pleased with the company, service and items purchased.